A simple form of display system is a character display system, in which only a font of alphanumeric characters is available to the user who can thereby choose to display textual and numeric information. An example of such a display system is the IBM (Trademark) 3270. At the other extreme is the display system, such as the IBM 2250, which can be programmed to display, not only alphanumeric information, but also extremely complex line figures useful in computer aided design. The advantage given by the flexibility of the latter system is often more than counterbalanced by the sophisticated programming required, which is beyond the resources of many potential users. Further, many users do not require all the facilities available and a more limited display system may be more suited to their needs. Such a system is provided by character graphics. In an alphanumeric display, it is usual to allot a fixed area of the display field to each character. We call such an area an image cell. In a relatively low capacity display system, there are twenty rows each of forty-eight image cells, giving a total capacity of nine hundred and sixty characters. A font of characters is held in a store and is selectively accessed to generate the display image. Character graphics is an extension of this technique whereby the font available to the user includes not only letters, numbers and typographic signs, but also line segments arranged at various angles, shading and other elements enabling the user to generate simple geometric or mathematical designs, such as graphs, histograms or outline maps, by arranging elements of the font side by side on the screen. We call each element of the font a character.
The display device most frequently used is a cathode ray tube but it is to be understood that other display devices, for example, liquid crystals, electroluminescent or electrochromic materials, or light-emitting diodes, can be employed. Although character graphics can be implemented on any type of display device presenting a suitable display area to the user, in this specification the system to be described by way of example uses a cathode ray tube. The display area will be called the screen.
In a monochrome display the cathode ray beam is caused to trace a raster of closely-spaced parallel lines on the screen. In response to video information consisting of a sequence of binary digits the intensity of the beam is modulated to produce a pattern of visible dots which make up the required display. The dots correspond to the discrete display elements of the matrix displays mentioned above. Video information is generated line by line in synchronism with the raster traced by the cathode ray beam by selecting the required characters from the font which is stored in a character buffer. Each character consists of an array of binary digits, for example, nine bits wide by twelve bits high. The arrangement of one bit within the cell defines the pattern of the character represented by the display cell. Since the character extends over a plurality of lines of the raster--in the example, twelve lines--it is necessary to copy out the character line by line in synchronism with the raster scan. Any of several well-established arrangements can be used to achieve this, of which the most common is to use a raster line counter in accessing the character store so that only the line of the character belonging to the line of video information being assembled is copied from the store. In certain models of the IBM 3270 display system (e.g. the IBM 3277 Display Station), a line buffer is used into which the characters forming a row of text are copied. The line buffer is a circulating shift register and from it the binary information forming the video information for each raster line is read.
A monochrome character graphics system is disclosed in U.S. Pat. No. 3,891,982 to Cheek et al. Although the image is constructed of image cells and a font of alphanumeric characters is provided, graphic symbols are built up as required by a decoding sequence of code words each defining a single vector. More than one code word is necessary when an image cell is to include a plurality of vectors and provision is made to accumulate and superimpose the newly generated vectors before supplying video information to the display device. It will be seen that, besides providing a color character graphic system, our invention is a considerable simplification over this Patent, which may, however, be taken as exemplifying, in its procedures for handling alphanumeric characters, a typical state of the art system.
The provision of color in an information display system requires much more data to be associated with each display position than one binary digit, since besides defining whether the cathode ray beam is to be brightened at a given position in the raster, the color of the spot must be defined. Since color is defined with reference to three primary colors, it is usual to use three bits to define a limited number of color combinations of the primaries to provide what has been found to be an adequate choice of colors for most display purposes. How the color is generated on the display forms no part of this invention which is concerned with the supply of color video information to the display device in an economical manner. Character graphics reduces the amount of color information required since the information relates to a whole image cell rather than to a single display position on the screen. But simply to nominate a single color for an image cell leads to an undesirable lack of flexibility and leads to problems when, for example, lines cross or when special effects such as colored backgrounds are required.
It is the aim of this invention to provide a simple and cheaply implementable means of achieving mixed color effects in a single image cell.
The line-crossing problem has been tackled in U.S. Pat. No. 4,016,544 to Morita et al, although not specifically in relation to color character graphics. The video signal is supplied selectively to three color registers, respectively red, green and blue information memories. The memories are read simultaneously to a display device. A one bit in a memory causes the corresponding primary color to be displayed. One bit in more than one memory causes a combination of the primary colors to produce a secondary color. The patent describes the problem involved in line crossing when colors are associated with graphic elements such as lines. If a red line is drawn and later a green line is drawn to cross the red line, with the system described in the patent as prior art, the crossing point of the red line is erased. According to the patent, the problem is solved by providing mask bits which allow the contents of the color registers to be changed only when the mask bit is a given value. The patent thus provides a form of data protection to permit a choice of whether, in the example given, the bit in the red information register representing the crossing point is to be changed thus permitting red or green or a combination of red and green is to be displayed at the crossing point. The patent requires for each display position or group of eight display positions, a mask bit for each color information register.